Method and device for producing polyesters, copolyesters and polycarbonates

ABSTRACT

In a process for producing polyester, the esterification of dicarboxylic acids and diols is carried out in at least one reaction stage, the prepolycondensation of the esterification product in at least one reaction stage and the polycondensation of the prepolycondensation product in at least one reaction stage. To increase the capacity of the apparatus forming one or more of the reaction stages in the production of the polyesters, at at least one of the reaction stages the supplied product system before it enters the reaction stage or within the reaction stage is divided into at least two partial streams and the partial streams entirely or partially separate from one another is passed through the reaction stage.

The invention relates to a method and a device for producing polyesters, copolyesters and polycarbonates by the esterification of dicarboxylic acids or dicarboxylic acid esters and diols or by the reesterification of dialkylcarbonates or diaryl carbonates with bisphenols in at least one reaction stage, prepolycondensation of the esterification or reesterification products in at least one reaction stage and the polycondensation of the prepolycondensation product in at least one polycondensation stage.

For the continuous production of polyethyleneterephthalate (PET) and its copolyesterd, terephthalic acid (PTA) or dimethylterephthalate (DMT) and ethyleneglycol (EG) as well optionally further comonomers as starting materials are used. PTA is mixed with EG and a catalyst solution to a paste and fed to a first reaction stage for esterification in which the esterification is carried out at atmospheric pressure or subatmospheric pressure by the splitting off of water. When DMT is used, the DMT melt and the catalyst together with the EG are fed to a first reaction stage for esterification and in this stage the reaction is carried out at atmospheric pressures with splitting off of methanol (MeOH). The split-off substances are fed together with distilled EG to a rectification column for the recovery of EG.

The recovered EG is fed anew to the esterification or is used to produce the paste. The product stream of the esterification or reesterification is fed to a reaction stage for prepolycondensation which as a rule is carried out under vacuum. The product stream of the prepolycondensation is supplied to a reaction stage for polycondensation. The obtained polyester melt is processed directly to fibers or to chips. In processes for the production of PET the esterification is carried out in two reaction stages formed by stirred vessels. With a plant capacity up to 400 metric tons per day, the prepolycondensation is carried out in an upright cascade reactor with a stirrer running along the bottom and with plant capacities up to 900 tons per day in two polycondensation stages with the first configured as a stirred vessel and the next as a horizontal cascade reactor. These horizontal cascade reactors are chambered at the sump side and can have a vertical perforated or ring disk on the stirrer on a horizontal shaft whose purpose is to provide a defined surface area.

The two esterification stages, two polycondensation stages and a polycondensation stage are provided in a plant in which the first three reaction stages are formed by stirred vessels and the last two reaction stages are configured as horizontal cascade reactors and provide a high stability and flexibility of PET production offering the best possibility for increasing plant capacity without uncontrolled increase in cost for requirement and structures (Schumann, Heinz-Dieter: Polyester producing plants: principles and technology. Llandsberg/Lech: Verl. ModerneIndusrie, 1996, P. 27 to 33).

In a plant comprised of two stirred vessels for esterification, a staged reactor for prepolycondensation and a horizontal cascade reactor for polycondensation, at relatively low cost one can obtain comparable stability and flexibility of polyester production although with the drawback that the dimensioning of the reactors for polycondensation and the polycondensation stage must be larger because of the increased vapor volume and the permissible transport dimensions are achieved already with average apparatus capacities.

It is the object of the present invention, starting from the afore-described state of the art, to provide a method and an apparatus for carrying out the method, which permits a significant increase in the capacity of an apparatus for producing PET with four reaction stages and a carryover alternatively of apparatus with larger fixed reaction stages to four reaction stages without an increase in the vapor loading and/or an increased danger of product entrainment with the vapors of prepolycondensation or polycondensation. Furthermore, in the production of polybutyleneterephthalate (PBT) from PTA and 1,4 butanediol (BDO) a critical vapor loading should be avoided in the vacuum-driven esterification stages.

These objects are attained in that the product stream supplied to at least one of the reaction stages prior to entering the reaction stage or within the reaction stage is divided into at least two partial streams and the partial streams, either completely separate from one another or partly separate from one another are passed through the reaction stages.

According to a further feature of the invention, the partial streams, preferably in equal volume, run through the reaction stages or sections of the reaction stages parallel to one another until they are combined in a common product outflow whereby the partial streams in the reaction stages are combined at the latest at the outlet and the volume rates of flow into and out of the reaction stages are controlled.

An alternative, in accordance with the invention resides in that the partial streams in the reaction stages for the prepolycondensation or polycondensation are passed in opposite direction and discharged through separate outlets.

In order to carry out the method it is within the scope of the invention to provide a cascade reactor or cage reactor which has two spatially separated segments traversed separately and in succession.

As a horizontal cascade reactor, preferably a disk stirrer reactor with perforated or annular disks is used.

With partial streams which flow toward one another, the inlets for the partial streams are each provided at one end of the disk stirrer reactor and the outlet for the product stream formed by the partial stream is provided in a middle region of the disk stirrer reactor.

In the case of a product line branching or a product differentiation by different viscosities, it is also conceivable to provide the inlet for the partial streams in the central region and the outlets for the partial streams at the ends of the disk stirrer reactor.

Instead of a disk stirrer reactor the use of an upright multistage reactor is possible in which the inlets are provided for the product stream in the head region and the outlet for the product stream resulting from the collection of the partial stream at the bottom and at least in an upper section partial stages are provided for the separate feed of the partial stream and a subsequent stage or a bottom chamber is provided for combining the partial streams.

A multistage reactor can be so provided that inlet and outlet are connected by a duct lying externally of the multistage reactor for the product feedback and in which a feed for the product ahead of the product and elements for dividing the product stream into partial streams are integrated.

In the drawing several embodiments in the form of flow diagrams of the method technology have been illustrated which are described in greater detail below.

FIG. 1 shows a plant comprised of four reaction stages for the production of PET and in which over duct (1) a paste-like mixture of PTA and EG together with a catalyst solution is continuously fed to a first reaction stage (2) configured as a stirred reactor and in which a temperature of 260° C. and a pressure of 1600 mbar (abs) are maintained. The product stream flowing from the first reaction stage (2) is fed by a line (3) to a reaction stage (4) configured as a stirred reactor and maintained at a temperature of 263° C. and a pressure of 1080 mbar (abs). The vapors formed by the esterification reaction are lead off by ducts (5, 6) to a rectifier column which has not been illustrated and in the latter are separated, into water and EG. The EG which is collected is directly fed back to the reaction stages (2, 4) and/or is used to form anew the paste-like mixture. The product stream having a degree of esterification of 97% is carried off by a duct (7) from the reaction stage (4) and is divided into two equal volumes in respective product partial streams which are fed via ducts (8, 9) for the purpose of prepolycondensation into two identical inlet stages (10, 11) of a vertical upright multistage reactor (12) with a stirred bottom compartment (13) at a pressure of 15 mbar (abs), and then to the following stages (14, 15) before being recombined in the bottom compartment (13) via duct (16) the product stream flowing from the multistage reactor (12) is fed to a disk stirrer reactor (17) with ring-shaped disks for polycondensation and is introduced at its front end while the finished polymer is discharged at its rear end via conduit (18) through conduit (19) of the multistage reactor (12) and through conduit (20) of the disk stirrer reactor (17). The volume required for operation is applied. To avoid droplet entrainment out of the multistage reactor (12), the stages (10, 11, 14, 15) can be chambered and provided with droplet separators. With the afore-described plant construction, critical vapor loadings are avoided even with high throughputs.

A further refinement of the method of the invention is shown in FIG. 2. Through line (21) the paste mixture comprised of PTA and EG together with the catalyst solution is fed for esterification into the first reaction stage (22) configured as a stirred container and the reaction product is fed by conduit (23) to a second reaction stage (24) configured as a stirred vessel, in which a vacuum partial stage (25) with a pressure of 550 mbar (abs) is integrated. By conduits (26, 27, 28), the vapors produced by the esterification are conducted to a rectifier and divided into water and EG, whereby the vapors flowing from the vacuum partial stage (25) at a temperature of 267° C. are comprised by means of a gas jet pump (29) operating with the vapors of the first reaction stage (22) has a drive vapor. The product stream derived via conduit (30) from the vacuum partial stage (25) is divided into two product streams of the same volume, from which one product partial stream passes via conduit (31) to the front side and the other product stream via line (32) to the rear side of a disk stirrer reactor (33) with perforated disks for the purpose of prepolycondensation, whereby the partial stream flow through the reactor axially from the exterior inwardly in two separate mirror-symmetrically identical compartments. The product streams are fed together in the middle plane of the disk stirrer reactor (33) and is discharged as a product with a temperature of 274° C. via conduit (34). Over conduit (35) the requisite vacuum is generated in the reactor and simultaneously the fibers formed by prepolycondensation are carried off for recovery of EG. The product stream flowing via conduit (34) is subdivided into two partial streams, one stream passing via conduit (36) and the other via conduit (37) to two disk stirrer reactors (38, 39) for polycondensation. The polycondensation product at a temperature of 282° C. is discharged via the conduits (40, 41) from the disk stirrer reactors (38, 39). The vacuum required in the stirred disk reactors are applied at the conduits (42,43). Two reactors serve on the one hand to maximize the overall plant capacity and on the other hand for polymer diversification and to provide a decentralized product separation for direct spinning.

According to FIG. 2A, it is also possible to feed the partial stream discharged through the conduit (36) to the front side and that discharged by the conduit (37) to the rear side of a single disk stirrer reactor (44) with perforated disks and ring disks and to conduct the partial streams axially from the exterior inwardly through two separate mirror symmetrically identical reaction chambers. The product stream resulting from the combined partial stream is discharged in the middle plane of the stirred disk reactor by conduit (45). Via conduit (46) the requisite vacuum is produced and the vapors formed by the polycondensation are carried off.

A further modification of the process of FIG. 2 is shown in FIG. 2B. In accordance with this Figure it is possible to produce from the product stream derived via conduit (34) from the prepolycondensation initially amorphous or partly crystalline PET chips in a granulation system (47) to feed these chips via conduit (48) to a crystallizer system (49) with a temperature of 210° C. and then subject the product as fed via line (50) to a solid polycondensation at (51) in an inert gas stream at a temperature up to 230° C. in an SSP reactor (SSP=Solid State Polycondensation) for example an upright tube reactor as is known. The finished chips are discharged from the SSP reactor via duct (52).

For the continuous production of polybutylenetere-phthalate (PBT), PTA and butandiol (BDO) are mixed together and the pasty mixture according to FIG. 3 is fed via line (3) into line (54) and the heat exchanger (55) provided therein. This flow is then broken into two product partial streams of equal volume rates of flow which are then fed to parallel but separate input stages (58) or (59) of a multistage reactor (60) through the conduits (56) and (57). In these stages the esterification takes place at a temperature of 245° C. and a pressure of 400 mbar (abs). The separately fed partial product streams are combined in the intermediate stage (61) and the product is discharged from the bottom compartment (62) via duct (63). A predominant proportion of this product stream is passed via duct (64) and line (65) for the recovery of the esterified product and feeding of the raw materials via line (65) in a feed back to the head of the multistage reactor (60) through line (54). The remaining portion of the product stream discharged from the multistage reactor (60) is passed by a line (65) to a reaction stage (66) in which the prepolycondensation is carried out at a temperature of 240° C. and a pressure of 20 mbar (abs). For polycondensation, the prepolycondensate is fed via line (67) to the disk stirring reactor (68) from which the finished PBT is discharged by a duct (69).

The mixture of water, tetrahydrofuran (THF) and BDO formed in the esterification reaction is treated in a rectifier column. The THF and water as the head product is fed to a THF recovery and BDO as the sump product is supplied anew to the multistage reactor (60). The vapors from the prepolycondensation stage (66) and from the disk reactor (68) are drawn off through the ducts (70, 71) to separate vacuum systems and are subjected to a artial condensation for the purpose of recycling the BDO. 

1. A method for the continuous production of polyesters, copolyesters or polycarbonates by the esterification of dicarboxylic acids or dicarboxylic acid esters and diols or by the reesterification of dialkylcarbonates or diarylcarbonates with bisphenols in at least one reaction stage, prepolycondensation stage of the esterification or reesterification product in at least one reaction stage and polycondensation of the precondensation product in at least one reaction stage, characterized in that in at least one of the reaction stages, the supplied product stream ahead of or within the reaction stage is subdivided into at least two partial streams and the partial streams entirely or partly are fed through the reaction stages separately from one another.
 2. The method according to claim 1, characterized in that the partial streams are fed together to a common product discharged upon traversing the reaction stages.
 3. The method according to claim 1, characterized in that the partial streams are fed segmentwise parallel to one another in the reaction stage in the direction of a common product discharge.
 4. The method according to claim 1, characterized in that the partial streams are fed through the reaction stages in equal volumetric flow rates.
 5. The method according to claim 1, characterized in that the partial streams are combined in the reaction stages at the latest at the outlet.
 6. The method according to claim 1, characterized in that the partial streams in the reaction stages for prepolycondensation or polycondensation are fed in opposite directions to one another through separate outlets.
 7. The method according to claim 1, characterized in that the quantity rate of flow of the product streams offset into or discharged from the reaction stages are controlled.
 8. A device for carrying out the method for continuous production of polyesters, by esterification of dicarboxylic acids or dicarboxylic acid esters and diols or by reesterification of dialkylcarbonates for diarylcarbonates with bisphenols in at least one reaction stage (2, 4, 22, 24, 25, 60), prepolycondensation of the esterification or reesterification in at least one reaction stage (12, 33, 66) and polycondensation of the prepolycondensation product in at least one reaction stage (17, 38, 39, 44, 68) whereby at at least one of the reaction stages the supply product stream before or within the reaction stage is divided into at least two partial streams and the partial streams entirely or partly are fed through the reaction stage (12, 33, 44, 60) wholly or partially separate from one another, characterized in that the reaction stage to which the partial streams are supplied (12, 33, 44, 60) is a cascade reactor or a cage reactor with at least two partially separate partial compartments traversed separately one after the other and arranged in order.
 9. The device according to claim 8, characterized in that the cascade reactor is a disk stirring reactor (35) with perforated or annular disks.
 10. The device according to claim 9, characterized in that partial streams flow toward one another through inlets for the partial streams at the ends of the disk stirrer reactor (35) and the outlet for the total product stream formed from the partial stream is at a middle region of the reactor.
 11. The device according to claim 9, characterized in that the directions of the partial streams are opposite one another and the inlet for the partial streams are at the middle region and the outlet for the partial streams at the ends of the disk stirrer reactor.
 12. The device according to claim 8, characterized in that the cascade reactor is a multistage reactor (12, 60) having at least two inlets (8, 9, 56, 57) for the product stream at a head region and an outlet for the product stream of the combined partial streams at the bottom and at least in an upper section parallel stages (10, 11, 58, 59) for feeding the partial stream and an additional stage (14, 15, 61) or a bottom compartment (13, 62) for combining the partial streams.
 13. The device according to claim 12, characterized in that inlet and outlets are connected through a recycle for the discharged product outside the multistage reactor (60) by means of a duct (63, 64, 54) with elements contained therein for dividing the product stream in duct (54) to separate inlets (58, 59).
 14. The device according to one of claim 8, characterized in that the outlet opening or the outlet openings for vapor discharge of the cascade reactor (12, 17, 33, 38, 39, 44, 60, 68) or of the cage reactor are as far as possible from the inlet opening or the inlet openings.
 15. The device according to claim 8, characterized by an esterification stage comprised of two stirred reactors (2, 4), a prepolycondensation stage comprised of one multistage reactor (12) and a polycondensation stage comprised of one disk stirring reactor (17).
 16. The device according to claim 8, characterized by an esterification stage comprised of two stirred reactors (22, 24, 25), a prepolycondensation stage comprised of one disk stirred reactor (33) and a polycondensation stage comprised of one disk stirred reactor (38, 39).
 17. The device according to claim 8, characterized by an esterification stage comprised of two stirred reactors (22, 24, 25), a prepolycondensation stage comprised of a disk stirred reactor (33), a granulator system (47) a crystallizer system (49) and a vertical tube reactor (51) forming a polycondensation stage.
 18. The device according to claim 16, characterized in that in the second esterification stage (24) a vacuum esterification stage (25) is integrated.
 19. The device according to claim 8, characterized by a multistage reactor (60) with an external product circulation as the esterification stage and which is followed by a prepolycondensation stage (66) and a disk stirred reactor (68) for polycondensation. 